You may need more calories. Protein immediately after your workout.
Try these leg exercises and move up to 3 sets instead of only 2. Always start your leg workouts with a warm-up set.
Squats
Leg (knee) extensions (quads)
Leg Curls (hammy's)
Go heavy to add mass. Go until complete and utter failure. Do not forget to use a machine, smith machine or a spotter for squat failure.
I imagine your key is going to be by adding calories. Try adding 250 daily calories too your diet each week until you are making gains.
-Rip